Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) devices are being developed towards two main directions. The first direction is for high voltage and ultrahigh voltage applications. That is, the MOSFET devices have rather high breakdown voltages while having relative low on-state resistance, and the MOSFET devices developed towards this direction generally have a very thick low-doped epitaxial layer for bearing high voltage. Therefore, the drift resistance in the epitaxial layer becomes a major factor in the on-state resistance. The second direction is for low voltage and ultralow voltage applications. That is, the MOSFET devices have very small on-state resistance and fast switching speed instead of long duration for withstanding high voltages.
In order to achieve small on-state resistance, low-voltage MOSFET devices require more and smaller cells to construct. Early low-voltage MOSFET devices were produced by adopting planar structures. However, the planar structure is inadequate for reducing the area of a single cell, and therefore does not facilitate in realizing very small on-state resistance. Now, the MOSFET devices are generally made by adopting a trench technology (the MOSFET devices made by this technology are called trench MOSFET devices). This trench technology can tremendously increase the density of the cells and can be good for decreasing the on-state resistance.
When fabricating the low-voltage MOSFET devices using the trench technology, the requirement for the low-voltage MOSFET devices to withstand voltages is relatively low. Thus, the epitaxial layer of the low-voltage MOSFET devices can be thinner or doping concentration can be higher, and the proportion from the drift resistance of the epitaxial layer in the on-state resistance decreases. The influence of the channel resistance to the on-state resistance also increases. Further, if the density of the cells is fixed, decreasing the channel resistance can effectively decrease the on-state resistance. Moreover, if the length of the channel is fixed, the channel resistance can be decreased by widening the width of the channel.
The trenches formed by the conventional process are generally strip-type or square-type trenches. For example, FIG. 1 shows a top view of strip-type trenches and contact hole in a MOSFET cell. As shown in FIG. 1, the cell includes two separated strip-type trenches 1 and 2 (referring to the shaded area) and a contact hole 3 between the strip-type trenches 1 and 2. Further, FIG. 2 shows a top view of a square-type trench and contact hole in another MOSFET cell. As shown in FIG. 2, the cell includes a square trench 4 which forms a circular channel and a contact hole 5 disposed in an area surrounded by the circular channel.
FIG. 3 shows a cross-sectional view taken along the line AA′ in FIG. 1. As shown in FIG. 3, the cell includes a substrate layer 6, an epitaxial layer 7, a dielectric layer 8, and a metal layer 9 superposed sequentially. The epitaxial layer 7 includes the strip-type trenches 1 and 2, a body region 11 located between the strip-type trenches 1 and 2, and a source region 10 in the body region 11. The contact hole 3 goes through the dielectric layer 8 and extends to the body region 11 in the epitaxial layer 7, and the contact hole 3 is used to connect the source region 10 with the metal layer 9. The height d from the bottom of the source region 10 to the bottom of the body region 11 is then the length of the channel. The total length of the dashed line along the edge of the trench (FIGS. 1&2) is the width of the channel.
Therefore, if the length of the channel is fixed, the width of the channel is determined by a strip-type trench or a square trench and it may be difficult to increase the width furthermore. Thus, it may be hard to decrease the channel resistance to reduce the on-state resistance under the conventional fabricating processes. The disclosed methods and systems are directed to solve one or more problems set forth above and other problems.